Gaseous substances are constantly emitted to the atmosphere from both anthropogenic sources, such as vehicles and industries, and natural sources, such as volcanoes. Many of these substances have an effect on human health and/or on the global climate, the protective ozone layer in the stratosphere or other environmental effects. In order to study such effects and to quantify different sources etc., it is important that there are suitable equipment and methods available for measuring emissions and flux of such gaseous substances. The principles of some different methods are described in e.g. WO 02/04902.
Measurement of gases from volcanoes are important not only for studying their environmental impact and their effect on atmospheric chemistry, but also with regard to geo-science, as the gases emitted from a volcano carry important information on the geophysical and geochemical status of the interior of the volcano, as well as to risk mitigation/assessment, since the gas emission and composition adds to our understanding of the volcanoes status.
In measurements of gases emitted from relatively well demarcated sources, such as discharges from industries and volcanoes, the emissions can be measured at, or at least relatively close to, the source. In such measurements methods based on scattered sunlight spectroscopy are useful. In principle, scattered sunlight spectroscopy involves the use of a spectrometer to record the light of the zenith sky, i.e. the sky straight above the spectrometer, wherein an integration is made over the whole of the cross section of the emitted air mass. This results in a spectrum of the zenith sky including the vertically integrated concentration of the gases which are present in the atmosphere. By moving the spectrometer in such a way that the vertical layer that is measured cuts the emitted gas mass, e.g. an emission plume from a source, it is possible, after geometrical correction for deviations in direction of movement from a direction perpendicular to the propagation of the gas mass and after correcting for background concentration, to determine the integrated concentration over a cross section of the emitted gas mass. The emission is obtained after multiplying this integrated concentration with the concentration-weighed wind speed
A scattered sunlight spectroscopy instrument that has been used extensively in studies of volcano emissions is the so-called COSPEC-instrument, which is an optical remote sensing instrument developed in the 70's. It consists of a spectrometer that receives scattered UV-light from a narrow solid angle of the blue sky, typically at zenith. SO2 has a characteristic absorption spectrum around 300 nm. By means of a mask correlation technique the instrument is able to derive the total column of SO2 molecules along the vertical path or column defined by the instruments field-of-view (FOV). Thus the vertically integrated number of SO2 molecules in a segment of a volcanic plume may be derived. By installing the instrument on a moving platform, and move the platform in a direction such that the instruments FOV cuts the volcanic gas plume, the total number of SO2 molecules in a cross-section perpendicular to the direction of propagation of the plume may be derived. Multiplying this number with the molecular weight and the wind-speed gives the mass-flux in kg/s. The COSPEC-instrument has the advantage that the measurements can be made remotely at distances of several km from the source and that mass flux can be derived. For these reasons it has become the most important technique for volcanic gas emission monitoring.
Technological development during the recent decades has resulted in sensitive and fast multi-channel array detectors, powerful computers, and algorithms for modelling of radiative transfer and accurate analysis of differential absorption spectra. This has led to an alternative to the COSPEC-instrument: a miniature fiber optic ultraviolet differential optical absorption spectrometer: the mini-DOAS, which is described in Galle B., Oppenheimer C., Geyer A., McGonigle A., Edmonds M. and Horrocks L., “A miniaturised ultraviolet spectrometer for remote sensing of SO2 fluxes: a new tool for volcano surveillance”. Journal of Volcanology and Geothermal Research, Volume 119, Issues 1-4, 1 Jan. 2003, Pages 241-254. The basic mini-DOAS system consists of a pointing telescope fiber-coupled to a miniature spectrograph. Ultraviolet light from the sun, scattered from aerosols and molecules in the atmosphere, is collected by means of the telescope (length 10 cm, diameter 3 cm) with a quartz lens defining a field-of-view of 8 mrad. Light is transferred from the telescope to the spectrometer by means of a 2 m long optical quartz fiber with 800 μm diameter. The spectrometer uses a 2400 lines/mm grating combined with a 50 μm slit, providing an optical resolution of ˜0.6 nm over a wavelength range of 245-380 nm. The software used for controlling the spectrometer as well as for evaluating the spectra is a custom-built program.
In a typical zenith sky measurement the mini-DOAS is carried by car or foot, and spectra are recorded whilst the instrument is moved under the volcanic gas plume in a direction approximately perpendicular to the plume transport direction. The platform position is tracked using a GPS receiver making it possible to geometrically correct for deviations in movement from the ideal direction perpendicular to the plume transport direction. In this manner the total number of molecules in a thin layer of the plume can be determined, and after multiplication with wind speed at plume height, the mass flow of the emission can be obtained, e.g. in kg/s. Besides being a cost-effective alternative to the COSPEC, the introduction of the mini-DOAS represented a major step forward in terms of field-operability, mobility and flexibility. The small size and power consumption of the device made possible measurements in hitherto inaccessible areas and situations.
The mini-DOAS has been further developed in that the instrument was coupled to a scanning device consisting of a quartz-prism attached to a computer-controlled stepper-motor, providing a means to scan the field-of view of the instrument over 180°, see Edmonds M., R. A. Herd, B. Galle and C. Oppenheimer, “Automated, high time-resolution measurements of SO2 flux at Soufrière Hills Volcano, Montserrat”, Bulletin of Volcanology, Vol 65, 578-586, 2003. In a typical measurement the instrument is located under the plume, and scans are made, from horizon to horizon, in a plane perpendicular to the wind-direction. Thus, instead of moving the measurement equipment under the gas plume, this version of the mini-DOAS is capable of scanning a plane in the sky from a stationary position. Of course, this feature simplifies the measurement considerably. A stepwise scan of the plane typically use a 10 seconds integration time with 7.5° angular resolution, providing a full emission measurement every 4 minutes. The scanning mini-DOAS provides time resolved measurement of the gas emission, making it possible to correlate gas emission data with other geophysical data, e.g. seismic signals. A similar approach but using a mirror instead of a prism in the scanning device has also been used for volcanic gas measurements, McGonigle A. J. S., C. Oppenheimer, A. R. Hayes, B. Galle, M. Edmonds, T. Caltabiano, G. Salerno and M. Burton, “Volcanic sulphur dioxide flux measurements at Etna, Vulcano and Stromboli obtained using an automated scanning ultraviolet spectrometer”, Journal of Geophysical Research, vol 108, No B9, 2455, 2003.
As mentioned above one of the main advantages of the scanning mini-DOAS is that it is a stationary instrument that does not have to be moved during the measurement. However, this also means that the emission can only be measured when the wind direction is such that the gas plume from the volcano is passing over the instrument within a certain angular interval around zenith. If the wind direction changes such that it falls outside this interval the gas plume will, at least partly, be intersected by the instruments field-of-view at too high zenith angles to facilitate a good measurement. Typically it will not be able to determine unambiguously the clean air background below the plume. An effect of this is that it becomes difficult to evaluate the integrated concentration in the gas plume which leads to an unreliable determination of the amounts of emissions in the plume. In order to solve this problem efforts have been made to improve the mathematical algorithms for calculating the area of such incomplete plume scans by assuming certain plume dispersion characteristics and applying meteorological dispersion models. Another, more practical way, to solve this problem is to move the instrument or to install a number of instruments around the emission source such that the measurement ranges of two adjacent instruments overlap.
Another problem related to this measurement strategy is caused by scattering of light in the lower part of the atmosphere. In the described measurement strategy it is assumed that all the light that reach the instrument originates from a volume above the gas plume to be studied. If the Sun is located at an angle off zenith, direct light from the Sun may be scattered by aerosols and molecules located between the instrument and the gas plume, into the field-of-view of the instrument. This part of the recorded light has never passed the gas plume and thus causes a “diluted” spectrum, giving rise to an underestimation of the gas content in the plume. This problem gets worse the more aerosols (haze, particles, ash, rain, etc.) there is in the lower atmosphere. It also gets worse when the plume is intersected at high zenith angles (close to the horizon) and when the distance between the instrument and the gas plume is long.
To calculate the emission using the above mentioned methods, a knowledge of the transport speed of the gas plume is necessary. This can be obtained either by meteorological modeling, by extrapolating meteorological measurements made elsewhere or by measurement. A method for measuring the transport speed of the gas plume directly has been developed using DOAS techniques. In this method two DOAS instruments, similar to the systems described above, are located under the gas plume and made to point in two different directions, one beam upwind and the other downwind the plume, along the plume propagation direction. A time series of total column variations are registered in both directions, and from the temporal delay in variations in the total column, the wind-speed can be calculated if the plume height is known. Instead of two separate DOAS instruments one single system may be used where a flipping mirror instead alternates the pointing direction of the system between two directions. Ideally the parameter to be measured is the wind speed at plume height, weighted by plume concentration. As this approach uses variations in the total column of SO2 to determine the plume speed, a weighting with concentration is automatically made. Examples of wind measurements using this approach at Etna Volcano on Sicily and Popocatepetl Volcano in Mexico are given in Galle B., M. Johansson, C. Rivera and Y. Zhang, “Dual-Beam mini-DOAS spectroscopy, a novel approach for volcanic gas emission monitoring”, EGU General Assembly, Vienna, 24-29 Apr. 2005.
One object of the present invention is to provide a method and a device for measurements of the type described above, which method and device exhibit improved properties compared to conventional instruments and methods, in particular with regard to changing wind directions.